Thermosensitive recording materials are known which use a colorant system wherein a dye, such as a leuco dye, in one layer of the material reacts, upon the application of heat, with another component, a so-called “developer” in order to give rise to a coloured product.
In a typical thermosensitive recording material layer assembly, the following layers are present, constructed in the following order:                a support (base) layer is provided e.g. a paper support;        on top of the support (base) layer, there may optionally be one or more “undercoat” layers which do not contain the (dye+developer) couple;        on top of the support (base) layer, or on top of the uppermost undercoat layer if one or more undercoat layers is/are present, is the thermal layer (thermosensitive coloring layer) containing the (dye+developer) couple; and        on top of the thermosensitive coloring layer, there may be one or more “protective” layers. The protective layer or layers separate(s) the thermosensitive coloring layer from the outside environment and the uppermost “protective” layer is, like the lowermost support (base) layer (unless the latter has itself a backing layer), in contact with the outside environment.        
FIG. 1 shows an illustrative, non-limiting example of a thermosensitive recording material layer assembly (1). In this example, on top of the support (base) layer (2) there is a single undercoat layer (3). Following this there is a thermosensitive coloring layer (4), followed by a protective layer (5). In other known embodiments, two (or more) protective layers may be present, which may be numbered (51, 52, . . . ).
Thermosensitive recording materials are in common daily use, for example, in the transport industry for train, aeroplane and city underground railway tickets. They are also used in other ticketing applications such as parking, museum, cinema and concert tickets, as well as for displaying information on industrially prepared perishable foods, and also for facsimile machines. As a consequence, many types of resistance, for example resistance against plasticizers, water, heat, oil and light have been the subject of continuous attempts to improve thermosensitive recording materials.
Among such improvements, it has been proposed to provide a protective layer by coating at the surface of a heat-sensitive color-forming layer with an aqueous emulsion of a resin having a film-forming capability and resistance to chemicals (Japanese published patent application JP-54-128347) or a water-soluble high-molecular weight compound such as polyvinyl alcohol (Japanese utility model published application 56-125354). These methods are capable to prevent the color formation in non-image areas or fading in image areas even if the heat-sensitive recording paper is placed in contact with overwrap films or exposed to oils.
However, for the protective layer using such a water-soluble resin, in many cases the water resistance is absent, and a commonly used method (JP 57-188392) is to cure the protective layer used in combination with a cross-linking agent. Specific examples of the crosslinking agent (the curing agent) are polyepichlorohydrin, glyoxal, glutaric aldehyde, melamine compounds, polyamide-polyurea resin, aziridine compounds, zirconium compounds and boric acid.
EP 2 829 409 proposes to improve properties such as plasticizer resistance through the use of a specific developer combination, a mixture of 2,4′- and 4,4′-dihydroxyphenylsulfone, the 2,4′-isomer being the major component.
JP-2015-086352 addresses the issue of instability of cross-linked polyvinyl alcohol (PVA). It was known to add cross-linking agents to increase water-resistance of PVA. Known organic cross-linking agents includes aldehydes (e.g. formaldehyde, glyoxal, glutardialdehyde, glyoxylic acid and salts thereof), epoxy compounds, amino resins (e.g. urea resin, guanamine resin, melamine resin), amine compounds (e.g. ethylenediamine, hexamethylenediamine, meta-xylenediamine, 1,3-bisaminocyclohexane), hydrazine compounds, hydrazide compounds (e.g. adipic acid dihydrazide, carbodihydrazide, polyhydrazide), polyvalent carboxylic acids, polyoxyalkylene diamines or polyamines, acid anhydrides, polyisocyanates, block isocyanates. Known inorganic cross-linking agents include boric acid, borate salts (e.g. borax), zirconium compounds, titanium compounds (e.g. tetraalkoxy titanate), silicon compounds which have reactive functional groups, aluminium compounds (e.g. aluminum sulfate, an aluminium chloride, an aluminium nitrate, etc.), phosphorus compounds. However, cross-linked polyvinyl alcohol (PVA) was known to be potentially subject to yellowing over time. The authors of JP-2015-086352 found that certain chelating agents could reduce this yellowing effect. In particular, experiments were performed to investigate the effect of ethyelenediamine tetraacetic acid (EDTA) and some related polyamino-polycarboxylic acid chelating agents on the color stability over time of films prepared from coating liquids prepared from. PVA with adipic acid dihydrazide added as cross-linking agent. It is described that the resin composite layer containing PVA, cross-linking agent and chelating agent can be used as a protective layer for a thermal recording material, but multi-layer thermal recording materials are not prepared experimentally, and no test of plasticizer resistance of such a multi-layer thermal recording materials is carried out.
JP-H08-282100 proposes adding epoxy compound to the thermal recording layer of a thermal recording material in order to improve preservability of a recording image. However, this generates colouring of the background. As a countermeasure, EDTA is added as an inhibitor of colour development. U.S. Pat. No. 3,442,682 describes the use of a chelating agent such as EDTA in the thermosensitive layer as inhibitor of color development, with a dye which is not a leuco dye.